Photo-absorbing materials form the basis for optoelectronics applications, such as photo-detectors or solar cells, where photons are absorbed and converted into electrical signals. Typically, the photo-absorbing material is laid down in a planar geometry. Many materials do not absorb photons efficiently and thick layers (100s of nanometers to 100s of microns) are required to absorb the majority of the incident photons.
The large-scale implementation of photovoltaic (PV) technology around the world would greatly benefit from further cost reductions in the manufacturing of solar modules. Moreover, it is important to identify new ways to reduce the amount of semiconductor material used in PV cells, particularly for those cells employing non-earth-abundant elements like indium (CuInGaSe or CIGS cells) or tellurium (CdTe cells). Whereas thin film PV cells offer a viable pathway to reduce fabrication costs and material usage, their energy conversion efficiencies can still be improved significantly by enabling them to harness a larger fraction of the incident solar photons. As a result, researchers are diligently searching for new approaches to dramatically boost the amount of light absorption per unit volume of semiconductor.
In addition to conventional anti-reflection coatings, reflective substrates, and textured surfaces, more advanced light trapping techniques based on resonant cavities, plasmonics, and photonic crystals have recently gained significant interest. The best imaginable photon management (PM) technology would effectively trap and/or concentrate light in a broadband, angle-independent, and polarization-independent fashion. Resonant PM structures have demonstrated significant promise, but their performance is typically limited by a fundamental trade-off between the attainable absorption enhancement and their operational bandwidth. Moreover, resonant structures tend to exhibit a strongly angle-dependent optical response and the resulting solar cells require bulky solar tracking systems to follow the sun's movement in order to maximize their daily energy output. The fabrication of more advanced PM structures that could mitigate some of these issues is typically expensive and the increased cost offsets the potential performance gains.
Nano-wires (or nano-rods) have been employed in connection with solar cells in the art. Typically, the nano-wires are disposed vertically on a substrate (i.e., a “bed of nails” geometry). Representative examples include US 2006/0207647, US 2007/0111368 and US 2008/0047604. Lateral illumination of nano-wires has also been considered, e.g., as in US 2009/0188552.